Melt butter and olive oil in a heavy–based frying pan over medium heat. Add the pine nuts and toast for a couple of minutes, remove with a slotted spoon and set aside. Add the onion, garlic and rosemary and sauté, stirring, until onion is soft. Add the anchovies, squashing them to melt. Add mushrooms, then tomato and season.

Using a mortar and pestle, pound pine nuts to a paste and add to sauce. simmer for 20 minutes, stirring regularly. If the sauce seems dry, add a little water.